The present invention relates to a process and an apparatus for at least partially extracting, in an open or closed circuit, a liquid from an aggregate, by evaporation in a stream of carrier gas, followed by separation of at least part of said evaporated liquid from said stream of carrier gas, in which the source of energy substantially providing the separation work is the pressure increase of the carrier gas provided by mechanical means driving the carrier gas against all head losses throughout the complete circuit. This separation work comprises expanding the high moisture contents stream to a lower pressure so as to effect cooling and resulting liquid condensation, separating resulting liquid condensate from the gas stream, recompressing the cold stream with retained latent heat of vaporization thereby further heating said stream, and discharging the resulting heated stream with the lowered moisture content at an appropriate section of the circuit. More specifically, the present invention relates to a method of putting to work the driving energy which provides the separative work, and of carrying out said separation.
In this specification, the word "evaporation" has a very broad meaning which includes the production of vapor within a carrier gas by any mechanism, for example a chemical reaction such as combustion. Similarly, the word "aggregate" has a very broad meaning which includes not only materials soaked with a liquid, such as wet wool, or wet or liquid materials to be dried such as wood, milk, etc., but also materials reacting with gas, i.e., by combustion, thus forming a vapor which is a constituent of said reaction gas mixture. The aggregate may thus be, for example, methanization gas, natural gas, burnable waste products, fuel oil, coal, peat, brown coal or burning in air. It may also be, for example, living materials such as plants growing in a greenhouse, or machinery from which seeping liquids are leaking under liquid or vapor form.
The terms "exergy" and "exergetic", relate to that fraction of heat which is convertible in a reversible way into mechanical work, as opposed to "anergy" and "anergetic" which relate to that fraction of heat which is not convertible into mechanical work. The calorific energy of substance is the sum of its exergy and its anergy. By extension, the word exergy used herein also may mean mechanical work or any other energy which is convertible into mechanical work without being subject to the limitation imposed by Carnot's principle.
The expression "acoustic condensation" relates to the process of transient additional condensation of vapor which occurs within the flow of a gas/vapor mixture undergoing an adiabatic quasi-isentropic expansion when the flowing fluid is submitted to a high intensity acoustic radiation.
A first main application of the present invention is the drying of wet products, mainly industrial drying, wherein it permits the recovery of water vapor and its latent heat from air/vapor mixtures, using robust, cheap and compact equipment with low energy consumption, i.e., a blower and a thermodynamic separator, or a combined blower/separator, instead of delicate and expensive apparatus such as the known types of heat pumps.
A second main application of the present invention is the abatement of acid gases, such as sulphur dioxide and nitrogen oxides, from flue gases produced by combustion in boilers, furnaces and incinerators, through condensation of associated water vapor, using said robust, cheap and compact equipment; the energy consumption of which is small and more than offset by the recovery of the latent heat of condensation of the water vapor.
A third main application of the present invention is the extraction and upgrading of the latent heat of condensation of a vapor in a gas/vapor mixture, using said robust, cheap and compact equipment. Said application includes heating, ventilating and air conditioning processes and installations.
A fourth main application of the present invention is the extraction, for recovery or disposal or any other purpose, of valuable and/or polluting gases and vapors contained in process or waste gases, including used air rejected in the atmosphere by industrial production plants, by gasoline refilling stations for cars, and the like, and also including automobile exhaust gases.
Other applications of the invention will emerge from and be apparent to those skilled in the art from the detailed description which follows.
The background art in the technical field of the present invention is exemplified by U.S. Pat. 3,854,300 (which is fully incorporated herein by reference thereto) and by European Patent Application 0,162,509. U.S. Pat. 3,854,300 describes a method of reducing the high moisture contents of a stream of gases by expanding said stream to a lower pressure such as to effect cooling and resulting water vapor condensation; separating resulting water condensate from the gas stream; recompressing the cold stream with retained latent heat of vaporization thereby further heating said stream; and then discharging the resulting heated stream with the lowered moisture content to the atmosphere.
As discussed in U.S. Pat. 3,854,300 , said moisture containing gas stream may be expanded in a turbine type expansion means providing power therefrom and such power is then utilized in the recompression of the gas stream. The method of U.S. Pat. 3,854,300 is a particular application of the Brayton cycle which may be used for heat pumping in drying systems, as described in the following reference: J. L'HERMITTE et P. CHEVALIER, "Cycle de Brayton et pompes a chaleur--Developpement d'un systeme de sechage", E.D.F. (Electricite de France) Bulletin de la Direction des Etudes et Recherches Serie A, No 1, 1981 pp. 17-20.
In the HERMITTE reference, paragraph 1, "Presentation du cycle de Brayton, describes the background art relating to the Brayton cycle used in industry, as illustrated by FIG. 1 and FIG. 2. Paragraph 2, "Applications possibles" discloses several categories of applications, as illustrated respectively by FIGS. 3 to 7. Paragraph 3, Recherche de 'application du cycle de Brayton au sechage des cereales" discloses a particular embodiment of a drying process using a Brayton cycle, in which the drying chamber is at atmospheric pressure and no heat exchanger is required functionally, although an additional heat exchanger with a very limited heat exchange surface may be used to improve the overall coefficient of performance (COP) of the process. The operational flowsheets of the processes are illustrated in FIGS. 8 to 10 of the document, and its practical performances are shown in FIGS. 11 to 13 of the reference. It is clear to one skilled in the art the performances are poor.
European Patent Application 0,162,509 discloses a drying process (and corresponding apparatus) which is substantially a particular application (with waste heat recovery) of the method described in U.S. Pat. 3,854,300. In the European Application, the expansion and most of the recompression of the stream of gases, take place in an aerodynamic separator having no moving parts. The separator is composed of a convergent-divergent subsonic nozzle in which inertial separation of the condensate takes place at or near the throat region of the nozzle. The driving energy required by this aerodynamic separator is supplied to the process by a blower which moves the main gaseous stream against all head losses throughout the complete circuit. Auxiliary means may be provided in the inlet plenum of the nozzle in order to initiate and enhance the condensation process. In a particular embodiment described in the European Patent Application, the expansion of the gaseous stream includes a steady component and an oscillatory component, the latter being obtained by the action, within the flowing fluid, of very intense sonic or ultrasonic waves. The effect of the oscillatory component of the expansion is, on one hand, to induce acoustic agglomeration of the droplets of condensate and on the other hand, to cause transient additional condensation by the particular mechanism described in lines 8 to 13, page 12 of the specification, this mechanism is called "acoustic condensation". The required acoustic energy is generated mechanically either by the flowing stream itself, or by appropriate transducers, or by the blower supplying the driving energy required by the process.
According to the above-referenced European Patent Application, the amount of the acoustic energy radiated by the sonic wave represents only a minor fraction of the driving energy required by the process, and the latter energy itself represents only a minor fraction of the mechanical energy produced by the expanding fluid in the convergent part of the nozzle.
The drying process (and corresponding apparatus) described in the above-referenced European Patent Application has thus the following inherent limitations:
(1) The maximum pressure ratio of the steady state component of the expansion in the aerodynamic separator cannot exceed an upper limit at which the velocity of sound is reached in the throat of the nozzle; for atmospheric air, this limit is about 1.6 and is not sufficient for enabling per se the self-initiation of condensation during the phase of expansion of the incoming stream;
(2) When expanding air nearly saturated with water vapor, at pressure ratios not exceeding the latter limit, auxiliary means are required to enable the condensation in the throat region to reach a level close to thermodynamic equilibrium;
(3) The temperature increase which can be achieved by the process between the outlet and the inlet of the air in the dryer chamber cannot exceed an upper limit of 25.degree. to 30.degree. C., and the condensate quantity which can be extracted by the process cannot exceed 10 to 12 gr .sub.H 20 per kg dry air when acoustic condensation is not used; and
(4) Acoustic condensation may raise the quantity of condensate at the expense of the additional energy consumption required to provide adequate acoustic energy radiation.
Summarizing the state of the art, the known processes and apparatuses in the field of the invention (1) either have poor energetic efficiencies and COP's, and therefore they are not economically feasible in general; or (2) only have limited applications.